1. Field of the Invention
The present invention relates to an electrophotographic apparatus.
2. Description of the Related Art
In general, an electrophotographic apparatus comprises a drum-shaped image carrier disposed in an apparatus casing for rotation and an image forming mechanism arranged around the image carrier. The image forming mechanism includes a charger for uniformly charging the surface of the image carrier and an exposure unit for applying a laser beam to the charged image carrier surface in accordance with print data or the like, thereby forming an electrostatic latent image on the surface. The mechanism further includes a developing device for applying a toner to the image carrier surface with the electrostatic latent image thereon, thereby developing the latent image, a transfer device for transferring the resulting toner image to a paper sheet, and a de-electrifier for de-electrifying the image carrier after transfer. These elements are arranged successively around the image carrier.
If the toner or paper dust adheres to or remains on the surface of the image carrier after the transfer of the toner image, part of the printable surface of the paper sheet to be printed next may often blacken, thus entailing defective printing. Accordingly, the electrophotographic apparatus is provided with a cleaning system for cleaning the surface of the image carrier after the transfer.
Conventional cleaning systems include mechanical and magnetic cleaning systems.
In the mechanical cleaning system, a blade is brought into contact with the surface of the image carrier so that the residual toner is scraped from the image carrier surface by the blade, and the scraped toner is collected into a waste toner receiving portion. Since this system is designed so that the residual toner is scraped off directly by means of the blade, the image carrier can be satisfactorily cleaned for a long period of time. If the blade vibrates or if the pressure of contact between the blade and the image carrier becomes too high during the cleaning operation, however, the surface of the image carrier can be easily damaged. Thus, satisfactory print quality cannot be maintained.
The magnetic cleaning system, which is formed integrally with the developing device, cooperates therewith to effect development and remove the toner remaining on the surface of the image carrier by magnetic force so that the removed toner is recovered in the developing device. Although the control of the magnetic cleaning system is more complicated than that of the mechanical cleaning system, the magnetic system can recover the residual toner for reuse without touching the image carrier. Accordingly, many of modern electrophotographic apparatuses use the magnetic cleaning system.
When the residual toner adheres firmly to the surface of the image carrier, however, it cannot be securely removed if the image carrier surface is cleaned by means of the magnetic cleaning system whose cleaning capacity is lower than that of the mechanical cleaning system. In such a case, the residual toner causes defective printing, such as partial blackening of the printable surface of the paper sheet. If the cleaning capacity of the cleaning system is lowered during use, in particular, defective printing is liable to be caused.
The cleaning capacity of the magnetic cleaning system cannot be adjusted during the operation of the electrophotographic apparatus. In order to eliminate defective printing caused during the operation of the apparatus, therefore, the operation of the apparatus must be interrupted to adjust the cleaning system, thereby increasing its cleaning capacity. Accordingly, the printing efficiency is lowered. This problem may possibly be solved by previously adjusting the cleaning system to the maximum cleaning capacity before the start of the operation of the apparatus. If this is done, however, lowering of the capacity of the cleaning system will be accelerated, so that the lifetime of the system will be shortened.